Methods, systems, and computer readable media for dual resonance frequency enhanced electrostatic force microscopy

Abstract

The subject matter described herein includes methods, systems, and computer readable media for dual resonance frequency enhanced electrostatic force microscopy. One method includes applying an alternating current (AC) bias and a direct current (DC) bias to an atomic force microscopy cantilever, wherein the AC bias has a frequency greater than a fundamental resonance frequency of the cantilever. The method further includes mechanically vibrating the cantilever at a frequency different from the frequency of the AC bias. The method further includes physically and electrostatically scanning a sample in the same pass using the cantilever while vibrating the cantilever and applying the AC and DC biases to the cantilever, and generating a topology image of the sample from the physical scanning and an electrostatic image of charged material under or on a surface of the sample from the electrostatic scanning.

What is claimed is: 
     
       1. A method for performing enhanced electrostatic and atomic force microscopy, the method comprising:
 applying an alternating current (AC) bias and a direct current (DC) bias to an atomic force microscopy cantilever, wherein the AC bias has a frequency greater than a fundamental resonance frequency of the cantilever; 
 mechanically vibrating the cantilever at a frequency different from the frequency of the AC bias and wherein applying the DC bias includes using the DC bias together with the AC bias to cause the cantilever to vibrate at the frequency greater than the fundamental resonance frequency of the cantilever; 
 physically and electrostatically scanning a sample in the same pass using the cantilever while vibrating the cantilever and applying the AC and DC biases to the cantilever, and generating a topography image of the sample from the physical scanning and an electrostatic image of charged material under or on a surface of the sample from the electrostatic scanning, wherein physically and electrostatically scanning the sample includes monitoring a change in cantilever vibration amplitude as a function of sample position. 
 
     
     
       2. The method of  claim 1  wherein the sample comprises a biological sample. 
     
     
       3. The method of  claim 2  wherein the biological sample comprises a protein deoxyribonucleic acid (DNA) complex and wherein the electrostatic image includes an image of DNA under or on the surface of the protein DNA complex. 
     
     
       4. The method of  claim 1  wherein the frequency of the AC bias substantially corresponds to the first overtone frequency of the cantilever. 
     
     
       5. The method of  claim 1  wherein the frequency of the AC bias substantially corresponds to an overtone frequency of the cantilever greater than a first overtone frequency of the cantilever. 
     
     
       6. The method of  claim 1  comprising monitoring an output signal produced by the scanning, wherein the output signal reflects changes in vibration of the cantilever induced by interaction between the AC bias and an electrostatic charge or surface potential of the sample. 
     
     
       7. The method of  claim 6  comprising separating the output signal into a phase signal and an amplitude signal. 
     
     
       8. The method of  claim 7  wherein the phase signal corresponds to local energy dissipation in the sample related to local polar properties of the sample due to dipole-dipole interactions, and the amplitude signal corresponds to electrical force and force gradient between the cantilever and the sample determined by surface charge density or surface potential of the sample. 
     
     
       9. The method of  claim 1  comprising monitoring an overtone frequency of the cantilever and adjusting the DC bias applied to the cantilever or substrate to optimize an amplitude of vibration and frequency shift of the first overtone frequency, such that the vibration amplitude is sufficiently high and the AC bias substantially corresponds to the steepest shoulder of the overtone frequency of the cantilever after the cantilever is in contact with the surface of the sample. 
     
     
       10. The method of  claim 1  wherein vibrating the cantilever includes mechanically vibrating the cantilever at a frequency substantially near a fundamental resonance frequency of the cantilever. 
     
     
       11. The method of  claim 1  wherein the cantilever comprises a doped silicon material. 
     
     
       12. The method of  claim 1  comprising grounding the sample by placing the sample on a thin atomically flat insulator attached to a conductive sample holder. 
     
     
       13. The method of  claim 12  wherein the thin atomically flat insulator comprises freshly peeled mica. 
     
     
       14. The method of  claim 1  comprising monitoring a frequency of the AC bias and/or adjusting the frequency of the AC bias such that the frequency of the AC bias substantially corresponds to an overtone frequency of the cantilever during scanning of the sample. 
     
     
       15. A system for performing enhanced electrostatic and atomic force microscopy, the system comprising:
 applying an alternating current (AC) bias and a direct current (DC) bias to an atomic force microscopy cantilever, wherein the AC bias has a frequency greater than a fundamental resonance frequency of the cantilever; 
 mechanically vibrating the cantilever at a frequency different from the frequency of the AC bias and wherein applying the DC bias includes using the DC bias together with the AC bias to cause the cantilever to vibrate at the frequency greater than the fundamental frequency of the cantilever; 
 physically and electrostatically scanning a sample in the same pass using the cantilever while vibrating the cantilever and applying the AC and DC biases to the cantilever, and generating a topography image of the sample from the physical scanning and an electrostatic image of charged material under or on a surface of the sample from the electrostatic scanning, wherein physically and electrostatically scanning the sample includes monitoring a change in cantilever vibration amplitude as a function of sample position. 
 
     
     
       16. The system of  claim 15  wherein the sample comprises a biological sample. 
     
     
       17. The system of  claim 16  wherein the sample comprises a protein deoxyribonucleic acid (DNA) complex. 
     
     
       18. The system of  claim 15  wherein the frequency of the AC bias substantially corresponds to a first overtone frequency of the cantilever. 
     
     
       19. The system of  claim 15  wherein the frequency of the AC bias substantially corresponds to an overtone frequency of the cantilever greater than a first overtone frequency of the cantilever. 
     
     
       20. The system of  claim 15  wherein the analysis module is configured to monitor an output signal produced by the scanning, wherein the output signal reflects changes in vibration of the cantilever induced by interaction between the AC bias and an electrostatic charge of the sample. 
     
     
       21. The system of  claim 20  wherein the analysis module is configured to separate the output signal into a phase signal and an amplitude signal. 
     
     
       22. The system of  claim 21  wherein the phase signal corresponds to local energy dissipation in the sample related to local polar properties of the sample due to dipole-dipole interaction, and the amplitude signal corresponds to electrical force and force gradient between the cantilever and the sample determined by surface charge density or surface potential of the sample. 
     
     
       23. The system of  claim 15  wherein the function generator is configured to apply an adjustable AC bias to the cantilever and as well to the lock-in amplifier as a reference signal. 
     
     
       24. The system of  claim 23  wherein the analysis module is configured to monitor an overtone frequency of the cantilever and adjust the DC bias to optimize an amplitude of vibration and a frequency shift of the first overtone frequency, such that the vibration amplitude is sufficiently high and the AC bias substantially corresponds to a steepest shoulder of the overtone frequency of the cantilever after the cantilever is in contact with the surface of the sample. 
     
     
       25. The system of  claim 15  wherein the atomic force microscope is configured to mechanically vibrate the cantilever substantially near a fundamental resonance frequency of the cantilever. 
     
     
       26. The system of  claim 15  wherein the cantilever comprises a doped silicon material. 
     
     
       27. The system of  claim 15  wherein the sample is grounded by placing the sample on a thin atomically flat insulator with a thickness around 50 micro meters attached to a conductive sample holder. 
     
     
       28. The system of  claim 27  wherein the thin atomically flat insulator comprises freshly peeled mica. 
     
     
       29. The system of  claim 15  wherein the analysis module is configured to monitor a frequency of the AC bias and/or adjust the frequency of the AC bias such that the frequency of the AC bias substantially corresponds to an overtone frequency of the cantilever during scanning of the sample. 
     
     
       30. A non-transitory computer readable medium comprising computer executable instructions that when executed by a processor of a computer control the computer to perform steps comprising:
 applying an alternating current (AC) bias and a direct current (DC) bias to an atomic force microscopy cantilever, wherein the AC bias has a frequency greater than a fundamental resonance frequency of the cantilever; 
 mechanically vibrating the cantilever at a frequency different from the frequency of the AC bias and wherein applying the DC bias includes using the DC bias together with the AC bias to cause the cantilever to vibrate at the frequency greater than the fundamental resonance frequency of the cantilever; 
 physically and electrostatically scanning a sample in the same pass using the cantilever while vibrating the cantilever and applying the AC and DC biases to the cantilever, and generating a topography image of the sample from the physical scanning and an electrostatic image of charged material under or on a surface of the sample from the electrostatic scanning, wherein physically and electrostatically scanning the sample includes monitoring a change in cantilever vibration amplitude as a function of sample position.